As supplies of lighter and cleaner feeds dwindle, the petroleum industry will need to rely more heavily on relatively high boiling feeds derived from such materials as coal, tar sands, shale oil, and heavy crudes, all of which typically contain significantly more undesirable components, especially from an environmental point of view. These components include halides, metals, unsaturates and heteroatoms such as sulfur, nitrogen, and oxygen. Furthermore, due to environmental concerns, specifications for fuels, lubricants, and chemical products, with respect to such undesirable components, are continually becoming tighter. Consequently, such feeds and product streams require more upgrading in order to reduce the content of such undesirable components and this increases the cost of the finished products.
In a hydroprocessing process, at least a portion of the heteroatom compounds are removed, the molecular structure of the feed is changed, or both occur by reacting the feed with hydrogen in the presence of a suitable hydroprocessing catalyst. Hydroprocessing includes hydrogenation, hydrocracking, hydrotreating, hydroisomerization and hydrodewaxing, and therefore plays an important role in upgrading petroleum streams to meet more stringent quality requirements. For example, there is an increasing demand for improved heteroatom removal, aromatic saturation and boiling point reduction. In order to achieve these goals more economically, various process configurations have been developed using primarily downflow or trickle bed reactors, including the use of multiple hydroprocessing stages as is disclosed, for example, in U.S. Pat. Nos. 5,522,983; 5,705,052 and 5,720,872. Downflow trickle bed reactors must be designed with a high liquid mass velocity (liquid flow per cross-sectional area) to achieve good contacting of the catalyst with the liquid. This requires the cross-sectional area of the reactor to be small and therefore limited as to the amount of catalyst that it can hold, without the reactor being prohibitively high (e.g., .gtoreq..about.100 ft.).